FFMatrix + Imgsizer on MAMP install of EE 1.6.8 = no display of PNG thumbs in control panel's publish form

What’s installed:

EE 1.6.8 Build: 20100121 Image Sizer (v.2.5.6) FieldFrame (v.1.4) nGen File Field 1.0 (Create high quality thumbnails = yes) FF Matrix 1.5 Simple Math (v.1.0.1)

I did the basic FF Matrix Gallery tutorial as described at Pixel & Tonic Everything works fine except for one thing:

What the problem is: GOOD: In the publish page, where the gallery matrix appears, when I upload JPG images, save, and then come back to edit the entry, the jpg image thumbnail appears okay. BAD: When I do the same thing using a PNG image, save, and then go back to the entry to edit it, the thumbnail does not appear. (see the image that’s attached to this post)

Viewing the source for the control panel’s publish page, there is a difference between the JPG and PNG image path for displaying the thumbnail image:

IMG SRC path to JPG:

IMG SRC path to PNG: http://baseurl.local/images/uploads/

(here is the path set in context; I’ve broken each item onto a new line and numbered the lines. Look at line 5:

1 <td class="tableCellOne ngen_file_field td first">
2 <div class='ngen-file-field-block'>
3 <div class='ngen-file-field-data'>
4 <a href="http://baseurl.local/images/uploads/microtrack2-front_1000.png" class='ngen-file-link' target='_blank' rel="noopener">_5 http://baseurl.local/images/uploads/_6 microtrack2-front_1000.png_7 </a>

Oh, and when I look to see whether the thumbnail image was created, it was. I see PNG files located in this directory: /Volumes/www/sitehome/images/uploads/thumbs/

But for whatever reason, the extensions aren’t building a path to them.

It doesn’t matter whether I have the setting on nGen File Field high quality or low quality.

Any ideas why the PNG thumbnail images won’t appear?